Conventionally, in the resistance film type input device, in order to know which of the X-Y coordinates are depressed, two transparent films, each for X- and Y-axis, are disposed overlapped on, for example, the surface of the cathode ray tube. Each resistance film is provided with an electrode at its end portion. A predetermined current adapted for the X-axis is previously made to flow from one electrode to the other, and if the film is depressed, then the voltage at that depressed point which corresponds to the current which had been flowing for the X-axis is sensed to the resistance film adapted for the Y-axis to detect that voltage from the electrode adapted for the Y-axis. Then, which point on the X-axis was depressed can be known by that voltage to detect the X-coordinate according to the voltage generated at the Y-axis electrode.
While the film is being depressed (the period of time during which the film is depressed once is typically several tens of milliseconds), a predetermined current adapted for the Y-axis is also made to flow to detect the Y-coordinate of the depressed point according to the voltage generated at the X-axis electrode. Thus, the X-Y coordinate of the depressed point is detected.
An input detection means of the conventional input device using the resistance film adopts a system in which the DC voltage is always applied to the Y-axis electrode to detect the depression. For this case, the temperature of the resistance film is elevated, and the resistance value of the portion of the resistance film where the temperature is elevated is changed with age and the coordinate deviation which is caused by the change of the detected voltage occurs due to the change of the resistance value in two years or so after it begins to be used. Therefore, it is replaced with new one in the cycle of about two years.
Further, the resistance film is not only resistance film, but also is comprised of two resistance films in order to detect X- and Y-axes, and also has a characteristic as a capacitor having a charging capacitance. Therefore, if only the pulse voltages is simply applied as the driving voltage, electric charges charged to the X- and Y-electrodes cannot escape, and the coordinate is detected by error by the residual voltage.
As described above, in the conventional resistance film type input device, since the DC current is constantly applied while the input is waited for (during the waiting time), the resistance film accumulates heat and the resistance value is changed with age causing the coordinate deviation. Therefore, it must be replaced with new one in the cycle of about two years.
Further, it also has a disadvantage that the coordinate is detected by error by the electric charges remaining on the Y-axis electrode and on the X-axis electrode.
It is, then, an object of the present invention to provide a resistance film type input device for improving the operation of adapting for each electrode.